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looseness of the coiling is shown by the free apex and the slight 

 development and late incoming of the contact furrow. 



It is, of course-, as has been stated above, practically impossible 

 in many series to get sufficient evidence to establish the agreement 

 of chronology with bioplastology. But there are here and there 

 series that show such an agreement, and give approximately com- 

 plete and positive evidence in favor of the descent of nautilian from 

 arcuate forms. But even if this agreement occurred in a smaller 

 number of series than it actually does, the evidence from the 

 morphology alone would be sufficient. It is not possible to explain 

 why the apex of the transitional forms with large umbilical perfora- 

 tions is so often free, or the existence of the larger umbilical 

 perforations themselves, or, in fact, any of the peculiarities of the 

 nepionic stage, which resemble those of radical forms, except on 

 the assumption that they have been derived from these same straight 

 or arcuate radicals through direct genetic connection. Thus, 

 although the chronological record may coincide with the bioplas- 

 tology only in a few series, these few become positive evidence of 

 the highest value, that confirms the inferences drawn from the testi- 

 mony of the bioplastology and outweighs any amount of negative 

 evidence derived from the incompleteness of the record. 



With these remarks, we can now pass on to the consideration of 

 the history of the impressed zone, and its mode of origin and 

 apparent history in different series. 



There are a number of orthoceran and arcuate forms that may be 

 cited as the radicals of the Tarphyceratidse. 



These, like the history of the transitions into Aphetoceras, are 

 almost complete, since in this last genus the curvature in the young, 

 until a late stage, is so slight that one is not absolutely certain 

 whether to consider that such a fragment as is figured on PL v, 

 Figs. 15-17, is really a part of a gyroceran shell or a fragment of a 

 cyrtoceran form that never coils. The position of the siphuncle, 

 section of the whorl and sutures make the young of these forms 

 genetically identical with the adults of such forms as Aphetoceras 

 Americanum, and on the other hand the full-grown characters and 

 large gyroceran coils, are closer in some species than in others and 

 the genus passes by insensible gradations into the more closely 

 coiled nautilian genus, Pycnoceras. 



This last has the large umbilical perforation and almost cylindrical 



